Oxide Ion Diffusion Research Articles

Long-Term Oxidation of Ti2AlC in Air and Water Vapor at 1000–1300°C Temperature Range

The long-term isothermal oxidation behavior of bulk Ti2AlC in air and 100% water vapor has been investigated in the 1000–1300°C temperature range. The kinetics, for oxidation up to 120 hours, follows a cubic rate law in both environments – air and water vapor. The kinetics was found to be slightly faster for hydrothermal oxidation compared to oxidation in air, especially below 1100°C. However, there is little variation between the activation energies for oxidation in air (approximately 279 kJ/m3) and in 100% water vapor (approximately 261 kJ/m3). Scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction show that oxidation of Ti2AlC forms mostly a continuous and stable layer of α-Al2O3, along with a thin surface layer of rutile-TiO2 in both environments. However, the thin TiO2 layer volatilizes by forming gaseous TiO(OH)2 in the presence of water vapor at high temperatures (>1200°C). At short-term oxidation and lower temperatures, diffusion of hydroxyl ions is proposed to be the rate limiting mechanism under hydrothermal conditions. Above 1100°C, the diffusion of oxide ions through Al2O3 layer becomes the rate limiting step and humidity has little effect on the overall oxidation kinetics of Ti2AlC.

Local structural analysis for oxide ion transport in La0.6Sr0.4FeO3−δ cathodes

The relationship between the local structural changes and the oxide ion diffusion in La0.6Sr0.4FeO3−δ was investigated. The oxygen vacancy concentration in La0.6Sr0.4FeO3−δ was varied by annealing under various oxygen partial pressures. Local structural changes of La0.6Sr0.4FeO3−δ with the introduction of oxygen vacancies were studied by the extended X-ray absorption fine structure (EXAFS) analysis. Oxygen vacancies are preferentially introduced near the La ions and local distortion around the oxygen vacancies is induced. The oxygen vacancy diffusion coefficient, Dv was calculated by means of electrical conductivity measurement. Dv decreased with increasing local distortion around the oxygen vacancy. Activation energies for Dv strongly depended on the bottle-neck size calculated from the result of EXAFS.

Effects of humidified atmosphere on oxygen transport properties in La 2Mo 2O 9

The effects of humidified atmosphere on oxygen surface exchange and diffusion in La 2Mo 2O 9 have been investigated. After annealing samples in D 2O vapour, the depth and line scan profiles of the OD - species showed the incorporation of hydroxyl groups on the surface and of diffusion into the bulk. The hydroxyl diffusion process appears to be different from that of oxygen diffusion, and might indicate the existence of ambipolar diffusion of oxide ions and hydroxyl species in La 2Mo 2O 9.

Synthesis and Impedance Spectroscopy of Ga-Doped La<sub>9.33</sub>(SiO<sub>4</sub>)<sub>6</sub>O<sub>2</sub> Apatite Oxide Ion Conductor

Apatite materials are interesting alternative solid oxide fuel cell (SOFC) electrolytes because of their open structure including tunnels for the diffusion of oxide ions and their good chemical stability. However, the conductivity was still relative low when it was used as SOFC in the intermediate operating temperature range. So the apatite-type lanthanum silicates/germanates and doping were widely researched for enhancing conductivity recently. This study reports the influence of Ga doping on conductivity of the apatite phase La9.33(SiO4)6O2 according to the formula La9.33+x/3(SiO4)6-x(GaO4)xO2 where 0  x  2. Samples were prepared by a sol-gel process. The conductivities of all compositions have been obtained by impedance spectroscopy analysis. The conductivity enhancement observed for moderate x values and illustrates further influenced by the tetrahedral cation sublattice and the La vacancies in optimizing the conductivity.

Continuum and Kinetic Monte Carlo Modeling of Oxide Ion Diffusion under Reducing Conditions at Platinum/Yttria-Stabilized Zirconia Interface

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Crystal Structure, Diffusion Path, and Oxygen Permeability of a Pr2NiO4-Based Mixed Conductor (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+δ

We have investigated in situ the crystal structure, oxygen diffusion path, oxygen permeation rate, and electrical conductivity of a doped praseodymium nickel oxide, Pr(2)NiO(4)-based mixed conductor, (Pr(0.9)La(0.1))(2)(Ni(0.74)Cu(0.21)Ga(0.05))O(4+delta) (PLNCG) in air between 27 degrees C and 1015.6 degrees C. The PLNCG has a tetragonal I4/mmm K(2)NiF(4)-type structure which consists of a (Pr(0.9)La(0.1))(Ni(0.74)Cu(0.21)Ga(0.05))O(3) perovskite unit and a (Pr(0.9)La(0.1))O rock salt unit in the whole temperature range. Both experimental and theoretical electron density maps indicated two-dimensional networks of (Ni(0.74)Cu(0.21)Ga(0.05))-O covalent bonds in PLNCG. Highest occupied molecular orbitals (HOMO) in PLNCG demonstrate that the electron-hole conduction occurs via Ni and Cu atoms in the (Ni(0.74)Cu(0.21)Ga(0.05))-O layer. The bulk oxygen permeation rate was high (137 mumol cm(-2) min(-1) at 1000 degrees C), and its activation energy was low (51 kJ mol(-1) at 950 degrees C). The Rietveld method, maximum-entropy method (MEM), and MEM-based pattern fitting analyses of neutron and synchrotron diffraction data indicate a large anisotropic thermal motion of the apical O2 oxygen at the 4e site (0, 0, z; z approximately 0.2) in the (Pr(0.9)La(0.1))(Ni(0.74)Cu(0.21)Ga(0.05))O(3) perovskite unit. Neutron and synchrotron diffraction data and theoretical structural optimization show the interstitial oxygen (O3) atom at (x, 0, z) (x approximately 0.6 and z approximately 0.2). The nuclear density analysis indicates that the bulk oxide-ion diffusion, which is responsible for the high oxygen permeation rate, occurs through the interstitial O3 and anisotropic apical O2 sites. The nuclear density at the bottleneck on the oxygen diffusion path increases with temperature and with the oxygen permeation rate. The activation energy from the nuclear density at the bottleneck decreases with temperature, which is consistent with the decrease of the activation energy from oxygen permeation rate. The extremely low activation energy (12 kJ mol(-1) at 900 degrees C) from the nuclear density at the bottleneck indicates possible higher bulk oxygen permeation rates in quality single crystals and epitaxial thin films.

Evaluation of oxide ion diffusivity in YAG ceramics

Transparent yttrium aluminium garnet (YAG) polycrystal can be used as a host material for a solid state laser. In the present study, the authors prepared completely transparent YAG ceramics from active powder. The optical and mechanical properties of YAG ceramics are very sensitive to point defects. In order to examine the relationship between these properties and point defects, it is first necessary to characterise the defects. The authors report the effect of doping on oxygen diffusivity in several YAG ceramics, doped with various elements. Although Nd doping slightly increases the oxygen diffusion coefficient and the number of oxygen vacancies increases with increasing Nd concentration in as sintered materials, the variation of the oxygen diffusion coefficient in well annealed samples is small. These results suggest that the oxygen vacancies exist in non-equilibrium. However, grain boundary diffusivity was not influenced by preannealing before the oxygen diffusion experiments. The oxygen defect structure in transparent YAG ceramics is also discussed.

Oxygen Permeation Properties of Co-Free Perovskite-Type Oxide Membranes Based on BaFe1 − yZryO3 − δ

Partially Zr-substituted membranes were developed as a Co-free oxygen permeable membrane. In order to stabilize the cubic perovskite structure, Fe sites in were partially substituted with . In the substitution range of , the cubic perovskite structure was stabilized even at room temperature. Among the membranes prepared, a material showed the highest oxygen permeation flux of (standard temperature pressure) at under an air/He gradient. The oxygen permeation flux was higher than that of partially Ce-substituted membranes reported previously. From the results obtained by chemical and scanning electron microscope analyses, it appears that the oxygen permeability for membranes was well correlated with the amount of oxygen defects in the lattice as well as the grain size. In addition, the oxygen permeation flux of the membrane was significantly increased after decreasing the thickness of the membrane from . For thin membranes , the thickness dependence of the oxygen permeability deviated from the Wagner equation, suggesting that the oxygen permeation of is controlled by not only bulk diffusion of oxide ions but also their surface reactions.

Diffusion Path of Oxide Ions in an Apatite-Type Ionic Conductor La9.69(Si5.70Mg0.30)O26.24

Densities of coherent-scattering lengths in oxide-ion conductor La9.69(Si5.70Mg0.30)O26.24 with an apatite-type structure (space group P63/m) have been determined by a whole-pattern fitting approach based on the maximum-entropy method (MEM) using neutron powder diffraction data measured at 25 and 1558 °C. The Rietveld refinement suggested an interstitial oxygen (O5) atom site (12i; x, y, z; x ≈ 0, y ≈ 0.22, z ≈ 0.65) at the periphery of the hexagonal axis. The oxide ions located at 2a (O4) and 12i (O3) sites are largely distributed parallel and perpendicular to the c axis, respectively. The densities of coherent-scattering lengths derived from the MEM analysis revealed that the O4 oxide ions located at the 2a site (0, 0, 1/4) diffuse to nearest-neighbor 2a sites along the [001] direction. This [001] diffusion path is linearly linked with that of neighboring unit cell, which forms a long-range, one-dimensional oxide-ion diffusion pathway extending along the c axis of the hexagonal P63/m framework. The prob...

Structural Disorder and Diffusional Pathway of Oxide Ions in a Doped Pr2NiO4-Based Mixed Conductor

MEM nuclear density analysis from neutron diffraction data measured in situ at 1015.6 degrees C has indicated the two-dimensional network of curved O2-O3-O2 oxide-ion diffusion paths on the (Pr,La)-O layer in a K2NiF4-type structured oxide-ionic and electronic mixed conductor (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+delta.

Grain size dependence of electrical properties of Gd-doped ceria

A Gd-doped ceria powder (GDC, Ce0.8Gd0.2O1.9) prepared from an oxalate precursor was hot-pressed at 1173-1273 K in Ar atmosphere and annealed in air at 1273-1473 K to control the bulk density and grain size. Complex impedance of GDC of the microstructures of 0.29-5.6 μm average grain size and 95-98% theoretical density were measured at 573-1073 K in air in the frequency range from 100 Hz to 2 MHz. The bulk conductivity (σi(b)) was independent of the grain size and the activation energy for the migration of oxide ions was 85-88 kJ/mol. The grain boundary conductivity (σi(g)) was far small as compare with σi(b) (σi(g)=1/40-1/700 of σi(b) at 573 K). The σi(g) was influenced by the average grain size and showed a minimum value at around 3 μm and gradually increased with decreasing grain size. However, the total conductivity decreased for smaller grain size because of the increased thickness of grain boundary of relatively low conductivity. The information of diffusion of oxide ions from the surface into GDC inside was also reported.

Electrochemical impedance analysis of solid oxide fuel cell electrolyte using kinetic Monte Carlo technique

Quantum calculations complemented with kinetic Monte Carlo (KMC) simulations were performed to simulate impedance measurements in yttria stabilized zirconia (YSZ). The purpose of this study is aimed at gaining insight into the oxide ion diffusion process and the space charge double layer at the electrode–electrolyte interface subject to applied alternating potentials, and the dependence of impedance and double layer capacitance on the thickness of the electrolyte. The first step involves density functional theory (DFT) calculations to obtain a migration energy barrier database for oxide ions in YSZ. KMC simulations were subsequently performed to simulate the movement of oxide ions using the database with potential energy corrections under applied alternating potentials at different frequency domains. At low frequencies, the simulations show that oxide ions have sufficient time to migrate across the electrolyte and accumulate at the electrode–electrolyte interface creating a double layer with a space charge. The thicker electrolytes require longer time to attain same number of oxide–ion vacancies at the electrode–electrolyte interface. The current has the same phase shift as the applied potential. At high frequencies, only local movements of oxide ions are observed; therefore, there is no space charge accumulated at the electrode–electrolyte interface. The current response is shifted 90° from the applied potential. The resistance and geometric capacitance extracted from simulated Nyquist plots are linearly proportional to the thickness, as expected. The double layer capacitance is thickness independent and is approximately three orders of magnitude higher than the geometric capacitance. The temperature dependence of the resistance, the geometric and double layer capacitance was observed to have an activation energy of 0.67, 0.08 and 0.12 eV, respectively, at 400–700 K. The activation energies of the conductivity at both electrolyte thicknesses are similar to the results obtained by the KMC simulations in the random walk process. The activation energies and the values of both geometric and double layer capacitances can be related to experimental data indicating the validity of the described method.

Relationship between Oxide-Ion Conductivity and Dielectric Constant in Ce0.9Sm0.1O2-.DELTA.

The relationship between electrical conductivity and dielectric constant was investigated for 10 atom%Sm doped CeO 2 (Ce 0.9 Sm 0.1 Ο 2-δ ), which is a typical oxide-ion conductor. Computer simulation clarified that anomalously large dielectric constant originated from the superimposition of both Debye-type dipole and interfacial polarization between electrolyte and electrode. The interfacial polarization was proportional to the inverse of the frequency, suggesting the accumulation of oxide ions at the interfacial sites with high-energy barriers due to the oxide-ion conduction. Ac conductivity (σ ac ) at high temperatures and high frequencies agreed with the σ dc value, but lowered in such a frequency region that the Debye-type polarization appeared. The activation energy for σ dc agreed with those for Debye-type polarization and interfacial polarization, suggesting that these thermal exciting processes originate from oxide-ion diffusion.

Elaboration and ionic conduction of apatite-type lanthanum silicates doped with Ba, La 10− xBa x(SiO 4) 6O 3− x/2 with x = 0.25–2

The main task in SOFC research is the reduction of the working temperature down to the range 700–800 °C. Apatite ceramics are interesting alternative SOFC electrolytes because of their open structure including tunnels for the diffusion of oxide ions and their good chemical stability. This study reports the influence of barium doping on the microstructural and electrochemical properties of the apatite phase La 10− x Ba x (SiO 4) 6O 3− x/2 with x = 0.25–2. This doping range allows theoretical stoichiometric and overcharged apatite (more than two oxide ions in the tunnel). X-ray diffraction results on fired pellets show total insertion of the barium in the apatite lattice with a linear increase of the lattice parameters following the barium rate. Ionic conductivities are measured under air by impedance spectroscopy between 400 and 900 °C. The results are compared with apatite phases doped with calcium and strontium on the La site.

Electrical degradation of porous and dense LSM/YSZ interface

Electrochemical cells formed by the interface between dense and porous lanthanum strontium manganate (LSM) and yttria stabilized zirconia (YSZ) were submitted to annealing temperatures varying from 1373 K to 1673 K for 200 h and studied by Impedance Spectroscopy (IS) in order to investigate how the high annealing temperature can modify the contact between LSM/YSZ and to which extension these changes influence the electrical behavior of dense and porous LSM electrodes before and after the formation of insulating phases. Up to 1473 K the annealing process did not lead to substantial electrical behavior modifications at the LSM/YSZ interfaces for both porous and dense electrodes. IS measurements show two capacitive semicircles, the best fitting of impedance data brings to an equivalent circuit constituted by a serial combination of the electrolyte resistance and two parallel combinations of a resistance and a constant phase element, CPE. The higher frequency semicircles, HF, were attributed to the diffusion of oxide ions from the interface LSM/YSZ to the oxide ion vacancies located at the electrolyte surface. The semicircle at lower frequency, LF, will be ascribed to the oxygen species adsorption and diffusion in the LSM. At 1473 K the only changes recorded are related with the sinterization process of the porous electrodes. Over of 1473 K, the resistance contributions increased largely, especially for porous electrodes, and one additional semicircle was observed. This semicircle was associated to the oxygen diffusion process at the new insulating phases formed from YSZ and LSM solid state reactions. Porous and dense electrodes exhibited different rates for the degradation process. The porous electrode degraded faster than the dense one, probably because of the morphological effects as grain growth and their coalescence during annealing at higher temperatures.

Electron Emission from Indium Tin Oxide/Silicon Monoxide/Gold Structure

A field-emission cathode based upon the increase in the number of allowed conductive energy states in the silicon monoxide (SiO) forbidden gap as well as the increase in the number of interfacial states by indium tin oxide (ITO) ion diffusion into the SiO layer and silicon ion diffusion into the ITO layer while deposition is verified experimentally for the first time. This relatively low-cost electron emitter consists of an ITO layer sputtered on Pyrex glass, followed by an E-beam-evaporated silicon monoxide layer. The emitting layer is gold, deposited on the top of the silicon monoxide. Using ITO as a ground plate, instead of other previously published materials, a significant electron emission enhancement occurred.

Preliminary Investigation of the Higher-Order Ruddlesden-Popper Phases for IT-SOFC Cathodes, Lan+1NinO3n+1 (n = 2 and 3)

The Ruddlesden-Popper phase, La2NiO4+δ, has generated much interest as an alternative cathode material to the conventionally used perovskite-based materials, La1−xSrxBO3 (B = Mn, Co), particularly in the intermediate temperature range (873–1073 K). This is largely due to a combination of good electrical conductivity with high oxide-ion diffusion coefficients in this temperature range.

Effect of stability and diffusivity of extra-framework oxygen species on the formation of oxygen radicals in 12CaO·7Al 2O 3

The thermodynamics and kinetics of oxygen radical formation process in a nanoporous crystal, 12CaO·7Al 2O 3, were examined by isothermal annealing experiments in dry atmospheres. It is clarified that the formation of oxygen radicals, O 2 − and O −, in the crystal occurs in the following three steps; outward diffusion of extra-framework oxide ions, its oxidation by absorbed oxygen molecules to form O − and O 2 − at the surface, and subsequent inward diffusion of the oxygen radicals. The instability of the extra-framework oxide ions is essentially the driving force for the process. Further, the analyses allow to predict the total oxygen radical content for a given annealing condition.

Formation of Oxygen Radicals in 12CaO·7Al2O3: Instability of Extraframework Oxide Ions and Uptake of Oxygen Gas

The thermodynamics and kinetics of oxygen radical formation in cages of a nanoporous crystal, 12CaO·7Al2O3, were examined during isothermal annealing in a dry oxygen atmosphere. The formation of oxygen radicals, O2- and O-, involves the outward diffusion of extraframework oxide ions, their oxidation by absorbed oxygen molecules to form O- and O2- at the surface, and subsequent inward diffusion of the oxygen radicals. The instability of the extraframework oxide ions provides the driving force for this process. Evaluating the radical formation enthalpy and entropy enabled us to determine the temperature and oxygen partial pressure dependences of equilibrium oxygen radical concentration. The rate-limiting process for the radical formation is not the surface reaction, but the total ionic diffusion process, in which the smaller diffusivity of O2- likely dominates the process.

High temperature properties of La 0.6Sr 0.4Co 0.8Fe 0.2O 3− δ oxygen nonstoichiometry and chemical diffusion constant

Oxygen nonstoichiometry and oxide ion diffusion constant of La 0.6Sr 0.4Co 0.8Fe 0.2O 3− δ were gravimetrically studied with a CAHN1000 thermal balance, at temperatures 973–1073 K, with P O 2 between 1 and 10 5 Pa. The weight relaxation was mainly determined by bulk diffusion, but also influenced by surface exchange reaction. Diffusion coefficients were obtained by fitting the experimental data to the solution of the diffusion equation. For log ( P O 2 / P 0)>−2 ( P 0=101325 Pa), it was found that the oxygen vacancy diffusion constant is almost a constant, D V≒1.0×10 −5, for log ( P O 2 / P 0)<−2; however, a rapid decrease was observed. The surface reaction rate constant, k chem, is two orders of magnitude higher than the chemical diffusion constant. It depends on oxygen partial pressure approximately in the way, k chem∝ P O 2 0.55.